Physicists describe how to make time-reversed light pulses

May 31, 2011 by Lisa Zyga feature
The band structure of a photonic crystal supports pulse propagation in opposite directions, as indicated by the red arrows. Scientists have described a scheme that uses this band structure to time-reverse pulses with 100% efficiency. Image credit: Sivan, et al. ©2011 American Physical Society

(PhysOrg.com) -- By taking advantage of the properties of periodic systems, physicists have described how to efficiently time-reverse ultrashort electromagnetic pulses. Since a time-reversed pulse evolves as if time runs backwards, time reversal eliminates any distortions or scattering that occurred at earlier times, regardless of the medium the pulse has propagated through.

With this advantage, the ability to time-reverse broadband pulses could have applications in medical ultrasound, , superlensing, ultrafast plasmonics, and biological imaging. Applications could even extend beyond optical implementations, since the scheme does not rely on any concept that is unique to light. This universality opens up possibilities for using time reversal in systems for which it is not currently accessible, such as .

The research was performed by Dr. Yonatan Sivan and Professor Sir John Pendry (a pioneer in invisibility cloaks), both of Imperial College London. The scientists have published their study in a recent issue of .

So far, time reversal has been successfully demonstrated for pulses of a relatively narrow spectrum. On the other hand, schemes that enable time-reversal of broadband pulses have required complicated techniques, making them difficult to implement and giving them low efficiencies.

The new scheme opens the way to the efficient reversal of truly few-cycle pulses with devices that are easy to fabricate and implement. It is based on dynamically tuning the wave speed in that contain “zero gaps,” which are band gaps with a zero width. As the scientists explain, band gaps are energy or frequency regimes where waves cannot propagate, and are a common characteristic of periodic systems such as photonic crystals. When a band gap has zero width, an incident pulse can propagate through almost perfectly instead of being reflected.

“The zero-gap structure has two features,” Sivan told PhysOrg.com. “First, it serves as an effective homogeneous medium, in the sense that it admits all the incident ; a standard photonic crystal, with a finite gap, would reflect most of the pulse. Second, the existence of two bands at close proximity allows one to transfer energy between them using relatively slow modulations.”

The simplest example of a perfectly symmetric zero-gap system is a one-dimensional photonic crystal with two layers, each exactly a quarter wavelength wide. Such structures are frequently used for anti-reflection coatings. By tuning the wave velocity in the medium in real-time, the researchers calculated that short pulses propagating inside the structure could be reversed with up to 100% efficiency.

“The earliest proposal for using dynamically-tuned photonic crystals for time-reversal was made by Yanik and Fan from Stanford University,” Sivan said. “Their scheme was applied only to pulses of a few tens of picoseconds long and required fairly complicated structures; yet, it served as an inspiration. In that sense, we believe that the greatest significance of our work is to overcome these two limitations – our scheme requires very simple structures and can be applied also to much shorter pulses.”

In the future, the researchers hope to extend and improve this method of time-reversed .

“The continuation of this research involves application of the ideas to other structures, preferably simpler and more efficient,” Sivan said. “Second, we would like to work towards an experimental demonstration of the scheme. Some of these directions are well underway.”

Explore further: Using antineutrinos to monitor nuclear reactors

More information: Yonatan Sivan and John B. Pendry. “Time Reversal in Dynamically Tuned Zero-Gap Periodic Systems.” Physical Review Letters 106, 193902 (2011). DOI:10.1103/PhysRevLett.106.193902

4.4 /5 (17 votes)

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User comments : 44

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Sean_W
1 / 5 (1) May 31, 2011
Uh... good.??.
El_Nose
4.2 / 5 (5) May 31, 2011
interesting article - but i understood very little -- could someone dumb it down... thanks
TabulaMentis
1 / 5 (6) May 31, 2011
Sounds like a hypothesis for the forerunner of a practical time machine that travels backward into spacetime.
CSharpner
not rated yet May 31, 2011
interesting article - but i understood very little -- could someone dumb it down... thanks

Ditto.
FrankHerbert
2.5 / 5 (11) May 31, 2011
Sounds like a hypothesis for the forerunner of a practical time machine that travels backward into spacetime.


Let me guess. You already invented one, and we aren't worthy? Right?
TabulaMentis
1 / 5 (9) May 31, 2011
Let me guess. You already invented one, and we aren't worthy? Right?
Is that the best you can do dummy?
SincerelyTwo
4.4 / 5 (7) May 31, 2011
Let me guess. You already invented one, and we aren't worthy? Right?

Is that the best you can do dummy?

Is that the best you can do dummy?
antialias
5 / 5 (5) May 31, 2011
Found the full paper here:
arxiv.org/PS_cache/arxiv/pdf/1104/1104.4635v1.pdf

(Disclaimer: I'm not claiming I fully understand it from a standing start so this ad-hoc analysis may be wrong)

I think it describes what happens to the amplitide envelope of an incident pulse on a special type of crystalline, periodic structure. After reflection the change in the envelope seems to occur in the opposite sequence from the what happened before reflection (i.e. it is 'time reversed'...this has nothing to do with going backwards in time. It's more like playing a record forwards and after 'reflection' getting the sound back 'backwards').

I can see where that could be immensely useful in ultrasound applications.
TabulaMentis
1 / 5 (3) May 31, 2011
After reflection the change in the envelope seems to occur in the opposite sequence from the what happened before reflection (i.e. it is 'time reversed'...this has nothing to do with going backwards in time. It's more like playing a record forwards and after 'reflection' getting the sound back 'backwards').
Only time will tell! What I am talking about may be fifty years away. P.S.: Don't do anything you would not want people to know about in fifty years.
TabulaMentis
1 / 5 (8) May 31, 2011
@Antialias:
Are not you the German who has been protesting against nuclear power? If so, then congratulations, because Merkel has decided to abandon nuclear power by 2022. Good job!
Royale
3 / 5 (2) May 31, 2011
I read it twice and still am not too sure either. It seems like "time-reversal" isn't the best name. The article implies that it's used in anti reflective coatings. They bounce light away from the source. Do they plan on totally looping the light here? Is that time reversal? I'm not usually confused on this forum, but physorg you got me this time.
Royale
3 / 5 (2) May 31, 2011
Sorry antialias, i posted before i saw your post there. That helps clarify things a bit. Thanks.
TabulaMentis
1 / 5 (3) May 31, 2011
Royale:
I enjoy thinking about the future ramifications of today's discoveries/ideas and how they can be applied to tomorrow, fifty and one hundred years from now.
FrankHerbert
2 / 5 (8) May 31, 2011
Royale:

Actually TabulaMentis likes to post in articles that deal with groundbreaking technology/theories and claim he came up with them 10 years ago but won't elaborate any further because people are mean to him and he'll show them!
Na_Reth
1 / 5 (1) May 31, 2011
oh gawd. can we please stop using the word time in science?
TabulaMentis
1 / 5 (6) May 31, 2011
Actually TabulaMentis likes to post in articles that deal with groundbreaking technology/theories and claim he came up with them 10 years ago but won't elaborate any further because people are mean to him and he'll show them!
No, it was twenty years ago. I enjoy watching ideas that I created back then and since then be validated by new ideas from others.
Royale
2.3 / 5 (3) May 31, 2011
FrankHerbert, I know.
Tabula, I think it's enjoyable too. I remember taking Ecstasy and smoking a lot of Hydro back in college. I had this idea that things at the atomic level work similarly to the galaxies in the universe. Tripped out on it for about 2 hours. Then later on Men In Black comes out and shows the Universe with the camera pulling out and out and out. No, the idea was never 'mine' in that I never owned it, but I did enjoy seeing that someone else had come up with the idea and put it into action.
TabulaMentis
1 / 5 (4) May 31, 2011
Tabula, I think it's enjoyable too. I remember taking Ecstasy and smoking a lot of Hydro back in college.
Now I really do understand your problem!
Royale
5 / 5 (1) May 31, 2011
That was temporary mind altering. Not permanent as we've seen in some examples in other posts...
mattbroderick
not rated yet May 31, 2011
FrankHerbert, I know.
Tabula, I think it's enjoyable too. I remember taking Ecstasy and smoking a lot of Hydro back in college. I had this idea that things at the atomic level work similarly to the galaxies in the universe. Tripped out on it for about 2 hours. Then later on Men In Black comes out and shows the Universe with the camera pulling out and out and out. No, the idea was never 'mine' in that I never owned it, but I did enjoy seeing that someone else had come up with the idea and put it into action.


Powers of 10 came out way before you did your drugs.
malapropism
5 / 5 (3) May 31, 2011
arxiv.org/PS_cache/arxiv/pdf/1104/1104.4635v1.pdf
I think it describes what happens to the amplitide envelope of an incident pulse on a special type of crystalline, periodic structure. After reflection the change in the envelope seems to occur in the opposite sequence from the what happened before reflection (i.e. it is 'time reversed'...this has nothing to do with going backwards in time. It's more like playing a record forwards and after 'reflection' getting the sound back 'backwards').

Thanks for the arxiv ref. From a quick read it looks as if your summary is essentially right. The critical thing they've done is make this reversal work for a broadband signal (instead of the previous narrow band-limited signalling) at very high efficiency (100%). This has important implications in noise reduction in data communications systems, especially if the actual devices are simple to make (meaning also cheap) as they claim.
malapropism
1 / 5 (1) May 31, 2011
And I just reported you spamming b@$tards to the moderators... less than 5 minutes after your post, HA!
Graeme
not rated yet May 31, 2011
Is this time reversal working by making the whole solid structure reflect at one instant of time so that the pulse suddenly goes into reverse? How does it prevent time reversing the time reversal?

I can see there could be an application in microscopy. Also it should be possible to reform an image from something heavily scattered, such as light through the human body, or reflected around a corner from a matte surface.

If the pulse can be delayed for an arbitrary length of time the applications will increase!
Telekinetic
1 / 5 (2) May 31, 2011
http://www.roches...?id=2544

Five years ago, the researcher in the above link claims his time-reversed pulse (on the backstroke),is faster than the speed of light. I think this older experiment is far more interesting.
hush1
not rated yet May 31, 2011
I wonder if phonons are capable of exhibiting this behavior?
I have no idea what a zero gap structure for sound represents.

Thks antialias. The clarification, even if ad-hoc, makes more sense than what is worded here.
Wulfgar
not rated yet Jun 01, 2011
So to clarify, the reflected light is precisely "undoing" what it did on the way to the source of reflection. Isn't that the photonic equivalent of a broken bottle reassembling itself before our eyes? Are there implications for our understanding of entropy and the arrow of time? Seems like there would be...
frajo
4 / 5 (4) Jun 01, 2011
To use a term like "time-reversed pulse" in this context is highly misleading. The non-scientific public cannot help but think of a true time-reversal and, years later, will feel lied to by scientists. And the non-professional scientific minded croud is confused.
So what's that lingo good for? Attracting money?
hush1
not rated yet Jun 01, 2011
lol
The latent wording leads you to such thoughts. And asks you not to stop at the "broken bottle reassembling itself before your eyes", but asks you additionally to pictured that the reassembled bottle contain no "imperfections" the original unbroken bottle might have possessed.
A most admirable mechanism. The subject of which is not the subject of the research paper.
hush1
not rated yet Jun 01, 2011
@Frajo
You see on hand by way of Wulfgar that it is good for the imagination. To contemplate the impossible..."of things that never were" and ask: "Why not?"
Of course not all wording harbors noble intention.
fixer
1 / 5 (1) Jun 01, 2011
Is this the mythical "reverse tachion pulse" that James T Kirk had so much trouble with?
It seems likely!
rawa1
5 / 5 (2) Jun 01, 2011
The animation bellow illustrates what happens with light during experiment and the principle of time-parity reversal. Sometimes the picture is worth of thousands of words...

http://www.aether...al_c.gif
antialias_physorg
5 / 5 (3) Jun 01, 2011
The non-scientific public cannot help but think of a true time-reversal and, years later, will feel lied to

Then the non-scientific public should probably go read something else. I think a certain amount of scientific literacy can be expected of someone perusing physorg (and possibly even a certain amount of initiative to research stuff further that piques one's interst without going off on a rant about the low quality of journalism).
Royale
not rated yet Jun 01, 2011
ouch, burn antialias_physorg haha. Agreed.
Also, mattbroderick you're right! I had forgotten about Powers of 10 (Although I didn't hear of it until after college). I guess that makes it a novel thought, just not a purely original one.
TabulaMentis
1 / 5 (2) Jun 01, 2011
The animation bellow illustrates what happens with light during experiment and the principle of time-parity reversal. Sometimes the picture is worth of thousands of words...

http://www.aether...al_c.gif
Is this Physorg member 'Zephir' again with his or her ideas about aether?
orgon
not rated yet Jun 01, 2011
Nope, you just didn't understand it.
Wulfgar
not rated yet Jun 02, 2011
I'm sure my imagination does get the better of me, though I do try to tether it to the earth with what I know and what I read here. The article the researchers published is called Time Reversal in Dynamically Tuned Zero-Gap Periodic Systems and the Physorg article states that,

"Since a time-reversed pulse evolves as if time runs backwards, time reversal eliminates any distortions or scattering that occurred at earlier times"

That sounds to my layman ears like the light pulse is reversing its sequence of events in time. If they are not precisely doing so, but only reversing some aspects of the light, then I can understand why people object to what the authors are claiming.
Wulfgar
5 / 5 (1) Jun 02, 2011
The article states that:

Since a time-reversed pulse evolves as if time runs backwards, time reversal eliminates any distortions or scattering that occurred at earlier times.

which pretty much connects the dots for the reader. I know enough about physics not to have naive views about time travel, or the limits of such kinds of research. And it isn't surprising to find out that a physorg article can be poorly written. That's partly why I feel free to share my thoughts and ask questions.
antialias_physorg
not rated yet Jun 02, 2011
"Since a time-reversed pulse evolves as if time runs backwards, time reversal eliminates any distortions or scattering that occurred at earlier times"

That sounds to my layman ears like the light pulse is reversing its sequence of events in time.


Well, the sentence says quite clearly that it "the pulse evolves AS IF ... "

The 'as if' is a pretty good clue that this is supposed to be an anaolgy.
Royale
not rated yet Jun 02, 2011
So basically we're doing the opposite of what a prism does?
Can we all agree that this might be the easier way to look at it? Of course it's more complicated, and has many potential uses but it sound to me like light de-scattering.
TheGhostofOtto1923
3 / 5 (2) Jun 02, 2011
which pretty much connects the dots for the reader. I know enough about physics not to have naive views about time travel, or the limits of such kinds of research. And it isn't surprising to find out that a physorg article can be poorly written. That's partly why I feel free to share my thoughts and ask questions.
I think they're mostly written very well and maybe we get sucked in sometimes by other detractors who don't understand the subjects and think it is the authors fault. Like antialias says, you have to know enough about the subject to know when the author is assuming that you do. Unlike tabulamental we have to know when to say ooh aah and then move on.
Telekinetic
1 / 5 (3) Jun 02, 2011
"I know enough about physics not to have naive views about time travel, or the limits of such kinds of research."

Time travel was once considered scientific heresy, and I used to avoid talking about it for fear of being labelled a crank.
"These days Im not so cautious."- Stephen Hawking
rawa1
1 / 5 (1) Jun 02, 2011
You will travel across time dimension locally whenever you're compressing and/or expanding something, cooling or heating, falling or raising in gravity field (at the latest case you're travelling in space-time, rather than just in time, but the principle remains the same).
dompee
1 / 5 (1) Jun 03, 2011
the article is quite clear, as stated by the select few, you have to have a grasp of the science otherwise the majority of the article will make absolutely no sense to you...so you grab a couple words like time reversal and totally misinterpret what research was done....some things in science cannot be "dumbed down" you have to have some background knowledge of the particular scientific field being concentrated on.....do yourselves a favor go back to college get yourself @ least a mastersdegree and then return to this article and you'l get it ;)
WestMan
not rated yet Jun 04, 2011
This article seems to describe resonant, "standing" waves. They might be light, microwave, etc., and they maintain their energy potential by pinging in a prescribed, restricted pattern. If this is the case, then I submit that this idea has been around for years and years. Why is this news now? Check out Dr. Paul LaViolette. He identifies this phenomenon, and relates that MASERS are used with it all the time - possibly even to keep craft aloft!

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